1. Field of the Invention
The present invention relates to a switched mode power supply and more particularly to a switched mode power supply that can be used in AC/DC converters and DC/DC converters which incorporates both voltage mode and current mode logic to provide a multi-mode switched mode power supply which optimizes the transient response time and output regulation relative to known switched mode power supplies.
2. Description of the Prior Art
Both linear and switched mode power supplies are known in the art. Such power supplies are normally known as converters and are used to provide a regulated source of DC power to a DC device. AC/DC converters are used to convert an unregulated source of AC power to a regulated source of DC power. DC/DC converters are used to convert an unregulated source of DC power to a regulated source of DC power.
AC linear converters provide linear output power as a function of input power. Such linear converters normally include a step down transformer, a half or full wave rectifier and a voltage regulator. DC linear converters simply include a regulator. Although such linear power supplies are able to provide a fairly stable source of DC electrical power, such power supplies are relatively heavy, bulky and inefficient.
As such, switched mode power supplies have been developed. Such switched mode power supplies normally have lighter weight and are more efficient than linear power supplies. Such switched mode power supplies may include a high frequency transformer, a high frequency switching circuit and a pulse width modulation (PWM) controller for controlling the switching circuit.
A switched mode power supply or converter should maintain its specified performance (i.e. regulated output voltage) during any changes in the input source and/or output load. By sensing those changes, the control logic of a converter generates and adjusts a train of high frequency pulses from the PWM controller to regulate the performance of the converter based those changes. Examples of such switched mode converters are disclosed in US Patent Application Publication Nos.: US 2003/0026115 A1; US 2003/0034762 A1; US 2003/0090246 A1; US 2003/0151928 A1; US 2004/0263139 A1; US 2005/0116695 A1; US 2005/0212500 A1; US 2005/0219870 A1; and US 2005/0231984 A1; all hereby incorporated by reference.
Different control circuits are known to be used to control such switched mode converters. Voltage-mode and current-mode control methods are the most popular ones. US Patent Application Publication No. US 2003/00347762 A1 discloses a voltage controlled switched mode converter. In such voltage controlled switched mode converters, the output voltage of the converter is typically sensed and compared with a reference voltage. The difference between the converter output voltage and the reference voltage is used to control the duty cycle of the PWM, which, in turn, is applied to a switching circuit. As such, any changes in the output voltage due to load current and/or input voltage changes are utilized to adjust the PWM width of the drive pulses in order to regulate the output voltage.
Voltage controlled converters are known to have excellent voltage regulation in order to provide a regulated output voltage in response to changes in the input voltage and the output load. Such voltage controlled converters can also maintain good regulation even at no load. Unfortunately, such voltage controlled converters manifest a slow transient response to input and output load changes. Moreover, such voltage controlled converters are generally not suitable for power converters configured with a push-pull topology due to the possible flux imbalance of the high frequency transformer.
As mentioned above, current controlled switched mode converters are also known. US Patent Application Publication US 2004/0263139 discloses such a current controlled switched mode converter. In general, such current controlled switched mode converters include two feedback loops: an outer feedback loop, which senses the DC output voltage and delivers a DC control voltage to an inner loop which senses the peak power switcher current and keeps them constant on a pulse-by-pulse basis. Such current controlled switched mode converter offer many advantages over voltage controlled switched mode converters, such as, solving the flux imbalance problem in converters configured with push-pull topology. Such current controlled switched mode converters also have relatively faster transient response times to both input voltage and output load changes relative to voltage controlled switched mode converters. Finally, such current controlled switched mode converters simplify feedback-loop stabilization relative to voltage controlled switched mode converters.
Unfortunately, such current controlled switched mode converters have relatively poor output regulation compared to voltage controlled switched mode converters, especially during light load conditions. As such, current controlled switched mode converters normally need a fixed minimum load to stabilize the control loop and maintain output regulation during light load conditions. In some applications, such as during conditions when the converter is connected to a battery as a load, the battery may be damaged and lose its recharge capability permanently due to it being discharged to an extreme low voltage for a long time by the minimum load when the input power to the converter is lost.
Another problem with such switched mode converters relates to electromagnetic compatibility (EMC) and thermal management. In particular, even though such switched mode converters are relatively more efficient than linear power supplies, such switched mode converters are noisy due to the switching and thus can result in electromagnetic interference. In addition, in certain applications, such as recreational vehicle (RV) applications, the output power requirements of the such switched mode converter is designed to provide all of the power to the DC loads in an RV vehicle including cabin lighting, furnace fan motors and the like. The power requirement of such loads is relatively substantial. As such, such switched mode converters generate a significant amount of heat that must be dissipated. As such, electromagnetic compatibility (EMC) and thermal management are also very important considerations in the design and implementation of a switched mode converter.
Thus, there is a need to combine the advantages of both the voltage controlled and current controlled switched mode converters to provide a switched mode converter with better performance than a voltage controlled or current controlled switched mode converter individually while at the same time takes into account EMC as well as thermal management.